Explanation of error Missing mandatory value

The property of the object doesn't specify a value where, because of the defined
multiplicity of the property, one is required. Where this error is reported, the
'lower' multiplicity is 1 or more. This error report is also produced when a value was
provided as a reference to an xmi:id, but the file did not contain the referent.
There will be an additional error report to this effect if there was no referent
('Referenced object not found').